1. Field of the Invention
The present invention relates generally to a smart antenna apparatus and method, and in particular, to an apparatus and method for forming forward link transmission beams of a smart antenna in a mobile communication system.
2. Description of the Related Art
In order to meet a rapidly increasing demand for mobile communication and provide various multimedia services to users, there is a great necessity to increase capacity of a forward link. Typically, frequency division multiple access (FDMA) and time division multiple access (TDMA) are used to secure as large a subscriber capacity as possible with the limited frequency bandwidth available. In FDMA technology, a given frequency bandwidth is divided into a plurality of frequency channels necessary for communication, so that subscribers each use unique frequency channels. However, in TDMA technology, each subscriber uses a single frequency channel only for a predetermined time slot assigned thereto. Also, code division multiple access (CDMA) has been proposed which uses the same frequency band but distinguishes subscribers by assigning different codes to the subscribers.
However, the method of increasing efficiency of a limited frequency band by these multiple access technologies has a limitation in accommodating many subscribers. In order to overcome the limitation, cellular technology has been proposed. Cellular technology refers to a mobile communication technology that divides a service area into a plurality of small regions, or cells, and uses the same frequency band at two cells sufficiently distanced from each other to increase the number of spatially distributed channels, thereby securing sufficient subscriber capacity. Moreover, it is possible to further increase base station capacity by sectoring a base station antenna. For example, a base station antenna is sectored by changing an omnidirectional antenna with a 360° radiation pattern into three directional sector antennas with a 120° radiation pattern. Particularly, in a CDMA system, if a base station antenna is sectored, noises from subscribers of other sectors are reduced, contributing to an increase in call capacity of the base station.
Such conventional omnidirectional antenna or sector antenna transmits both a common channel signal and a transmission channel signal to a mobile station through a single common beam. The common channel signal includes a pilot channel signal, a synchronization channel signal and a paging channel signal, all of which must be transmitted from a base station to all mobile stations in a corresponding cell. The transmission channel signal refers to a traffic channel signal that must be transmitted to a particular mobile station. A considerable amount of radiation energy is wasted except the radiation energy transmitted to a particular mobile station, e.g., when a particular signal such as the transmission channel signal is transmitted to the particular mobile station in the same manner as the common channel, rather than when a predetermined signal such as the common channel signal is transmitted from a base station's transmission antenna to all mobile stations. In addition, such radiation energy acts as an interference signal to other mobile stations except the corresponding mobile station.
Therefore, if it is possible to transmit a transmission channel signal in a direction of a particular mobile station by certain means, it is possible to maintain high call quality while maintaining low transmission power and reducing interference signals to other mobile stations, thereby contributing to an increase in call capacity. An antenna based on such a concept is an adaptive array antenna, also known as an intelligent antenna or smart antenna.
A smart antenna system refers to an intelligent antenna system which can automatically change its radiation beam pattern in response to a predetermined signal environment. The smart antenna system adopts a technology for arranging a plurality of antenna elements in a specific form and multiplying an output of each antenna element by a complex weight, thereby forming an antenna beam in a direction of a desired mobile station.
Such a smart antenna system is a technology that can be widely used in a mobile communication field. Herein, however, the smart antenna system will be described with reference to a CDMA cellular mobile communication system. In addition, the smart antenna system is a technology in which a base station receives only a signal transmitted from a desired mobile station, in a reverse link, and concentrates transmission power only to a desired mobile station, on a forward link. Herein, the smart antenna system will be described on the assumption that a forward link transmission beam is formed.
A method for forming a forward link transmission beam of a smart antenna in a CDMA mobile communication system is disclosed in U.S. Pat. No. 6,108,565, which is incorporated herein by reference. The disclosed method calculates forward link transmission beam forming information by estimating an angle of arrival (AOA) and a time of arrival (TOA) from signals received at an antenna array of a base station from mobile stations. In addition, the patent discloses a method for forming a forward link transmission beam for each mobile station according to the calculated AOA and TOA of a received signal. That is, a common channel signal is transmitted through a wide beam, i.e., common beam, while a transmission channel signal for each mobile station is transmitted through a narrow beam, i.e., transmission beam, according to the calculated forward link transmission beam forming information. A beamwidth of the narrow beam is determined according to the distance between a mobile station and a base station. As the distance becomes shorter, the beamwidth becomes wider, while as the distance becomes longer, the beamwidth becomes narrower. In addition, a beamwidth of the narrow beam for the transmission channel signal is controlled according to a frame error rate (FER) reported over a reverse link.
A description of a method for forming a forward link transmission beam can be separately made with reference to one case where only a common pilot channel is provided to all mobile stations in a cell and another case where a dedicated pilot channel is provided to each mobile station so that each mobile station can easily perform coherent detection. In the latter case where the dedicated pilot channel is provided, since the dedicated pilot channel and the transmission channel use the same transmission beam, phase matching between both channels is guaranteed. However, in the former case where only the common pilot channel is provided, since the common pilot channel and the transmission channel use different forward link transmission beams, phase mismatching occurs between both channels. The phase mismatching has a different aftereffect according to a modulation scheme. Generally, a mobile communication system uses a modulation scheme of BPSK (Binary Phase Shift Keying) or QPSK (Quadrature Phase Shift Keying), commonly called “MPSK (Multiple Phase Shift Keying).” When the MPSK is used as a modulation scheme, a phase difference between a common channel signal and a transmission channel signal must be minimized in order to minimize a bit error rate (BER). That is, the phase mismatching must be minimized to obtain desired call quality. Therefore, in a general mobile communication system, there is a necessity to minimize the phase mismatching.
In the CDMA mobile communication system, a signal from one user acts as an interference signal to another user, so the interference must be well controlled in order to increase channel capacity. In particular, as demand for data communication having higher power than voice communication has increased recently, the inference problem becomes more significant. The smart antenna system has been proposed to drastically reduce the interference signals by forming a forward link transmission beam so that a transmission channel signal is transmitted in a direction of a desired particular mobile station. Actually, however, a part of the transmission channel signal transmitted to the desired mobile station is provided to the other mobile stations inside and outside a cell, causing undesired interference. However, such inference is not considered in the forward link transmission beam forming method disclosed in the U.S. Pat. No. 6,108,565.
Meanwhile, if a beamwidth of a transmission beam is increased to be as wide as a beamwidth of a common beam in order to minimize phase mismatching, interference to other mobile stations is increased. In contrast, if the bandwidth of the transmission beam is decreased in order to minimize interference, the phase mismatching is increased. That is, since the two conditions have a trade-off relation, it is necessary to consider the two conditions together in order to form an optimal transmission beam.